Writing Device Drivers

General Flow of Control

Assuming no transport errors occur, the following steps describe the general flow of control for a read or write request.

  1. The target driver's read(9E) or write(9E) entry point is invoked. physio(9F) is used to lock down memory, prepare a buf structure, and call the strategy routine.

  2. The target driver's strategy(9E) routine checks the request. strategy() then allocates a scsi_pkt(9S) by using scsi_init_pkt(9F). The target driver initializes the packet and sets the SCSI command descriptor block (CDB) using the scsi_setup_cdb(9F) function. The target driver also specifies a timeout. Then, the driver provides a pointer to a callback function. The callback function is called by the host bus adapter driver on completion of the command. The buf(9S) pointer should be saved in the SCSI packet's target-private space.

  3. The target driver submits the packet to the host bus adapter driver by using scsi_transport(9F). The target driver is then free to accept other requests. The target driver should not access the packet while the packet is in transport. If either the host bus adapter driver or the target supports queueing, new requests can be submitted while the packet is in transport.

  4. As soon as the SCSI bus is free and the target not busy, the host bus adapter driver selects the target and passes the CDB. The target driver executes the command. The target then performs the requested data transfers.

  5. After the target sends completion status and the command completes, the host bus adapter driver notifies the target driver. To perform the notification, the host calls the completion function that was specified in the SCSI packet. At this time the host bus adapter driver is no longer responsible for the packet, and the target driver has regained ownership of the packet.

  6. The SCSI packet's completion routine analyzes the returned information. The completion routine then determines whether the SCSI operation was successful. If a failure has occurred, the target driver retries the command by calling scsi_transport(9F) again. If the host bus adapter driver does not support auto request sense, the target driver must submit a request sense packet to retrieve the sense data in the event of a check condition.

  7. After successful completion or if the command cannot be retried, the target driver calls scsi_destroy_pkt(9F). scsi_destroy_pkt() synchronizes the data. scsi_destroy_pkt() then frees the packet. If the target driver needs to access the data before freeing the packet, scsi_sync_pkt(9F) is called.

  8. Finally, the target driver notifies the requesting application that the read or write transaction is complete. This notification is made by returning from the read(9E) entry point in the driver for character devices. Otherwise, notification is made indirectly through biodone(9F).

SCSA allows the execution of many of such operations, both overlapped and queued, at various points in the process. The model places the management of system resources on the host bus adapter driver. The software interface enables the execution of target driver functions on host bus adapter drivers by using SCSI bus adapters of varying degrees of sophistication.